The present invention relates to an injection molding apparatus and in particular, to an improved nozzle arrangement which permits the mold core to have an increased cooling capability and increased strength and which allows the fabrication of smaller diameter plastic articles without dangerously sharp edges.
Injection molding systems for forming plastic articles typically use hot runner nozzle arrangements to inject molten plastic material into a cavity space. In one type of hot runner nozzle arrangement, a heating element is placed near a center gate or an edge gate through which molten plastic material flows into the cavity space. This type of nozzle arrangement helps maintain the plastic material in the desired molten condition. U.S. Pat. No. 4,768,945 to Schmidt et al. exemplifies such a nozzle arrangement. As shown therein, the nozzle arrangement may include an inclined nose portion for housing a heating element for maintaining the plastic material in a molten condition and a hook-shaped passageway for supplying the molten plastic material to an edge gate.
Some molded plastic articles, such as cups and buckets, have molded lip or rim portions forming a top surface on the article. One of the more serious problems associated with the molding of these articles is the formation of unwanted sharp edges along the top surface and/or the outer edge of the lip or rim portion. This problem has been addressed by using a rim gating technique wherein molten plastic material is first injected into the inside edge of the rim defining portion of the cavity space and then permitted to flow down along the sidewall defining portions of the cavity space. This technique not only addresses the problem of sharp edges but also addresses other undesirable molding problems such as core shifting. U.S. Pat. No. 5,145,630 to Schad, assigned to the assignee of the instant application, illustrates this type of rim gating technique.
While successfully dealing with the core shifting and the edge problems, the Schad system is limited in the size of the articles which it can mold. In particular, it cannot be used to mold articles having an effective diameter below 150 mm. This is in part due to the fact that smaller diameter articles require the mold core to have an increased cooling capability. In the Schad design, the cooling capability of the mold core is limited by the nozzle arrangement. For example, as shown in FIG. 1 herein, which is FIG. 8 in the Schad patent, there exists a dimension A between the hot runner nozzles used to inject molten plastic material into the cavity space. This distance is all that is available for providing cooling means such as cooling inlets, cooling conduits and the like. While it is possible to add cooling means in the space A in the Schad design, the net effect is a reduction in the overall strength of the mold core portion--a reduction which could result in the breakage of the mold core portion.
Accordingly, it is an object of the present invention to provide an injection molding apparatus having an improved nozzle arrangement which allows the fabrication of smaller articles.
It is a further object of the present invention to provide an injection molding apparatus having an improved nozzle arrangement as above which allows the cooling capability of the mold core portion to be enhanced without sacrificing the overall strength of the mold core portion.
It is still a further object of the present invention to provide a rim-gated or edge gated injection molding apparatus having an improved nozzle arrangement as above which eliminates sharp edges along the top surface or outer edge of a molded article.
These and other objects and advantages of the present invention will become clearer from the following description and drawings wherein like reference numerals depict like elements.